Public security refers to the stable external environment and order required by the society and the public for normal life, work, study, entertainment and other activities, and refers to the safety of most people's lives, health and public and private property. According to the provisions of the emergency response law of the people's Republic of China, the public security emergencies involve four categories: natural disasters, accident disasters, public health events and social safety events.
In the international community, public security is also the focus of governments and relevant organizations. Especially in the developed countries such as the United States and Japan, the relevant research started earlier, and its emergency management and response methods were more abundant, and the legal policies and institutional mechanisms were relatively perfect.
The emergency management of public security emergencies is going through the process from simple disaster rescue to comprehensive disaster prevention and mitigation and relief. The experts and scholars in China have carried out in-depth research around the related problems and technologies. Public safety and emergency management cannot be separated from the support of modern science and technology. The national key R & D plan "public safety risk prevention and emergency technical equipment" launched and implemented in 2016 is the top-level planning of relevant national departments around the development of public security technology. At present, many achievements have been made in equipment, technical standards, software platform and other aspects.
Generally speaking, NDT technology is directly used to serve the public safety of cities. The safe operation of lifeline engineering, major infrastructure and special equipment in the city can not be separated from the protection of NDT technology. In the project set up in the past few years, the key project of the national key R & D plan "public safety risk prevention and control and emergency technical equipment" includes the research on the safety status inspection and evaluation technology of chemical equipment and facilities, oil and gas long-distance transmission pipeline and storage and transportation facilities, urban underground comprehensive pipe gallery, regional comprehensive transportation infrastructure, pressure bearing and mechanical and electrical special equipment.
According to the function of NDT technology and the characteristics of public safety emergency management, NDT technology can be applied to risk assessment, monitoring and rescue disposal. The single or irregular defect and performance test of special equipment such as pressure vessel, elevator, road and bridge, industrial facilities, oil and gas pipelines, water supply and gas pipelines by nondestructive testing technology is actually a process of risk assessment. The safety status of the equipment can be obtained according to the inspection results. The regular or continuous detection of these facilities and equipment belongs to the scope of monitoring and monitoring. Typical examples include structural health monitoring of infrastructure such as buildings and bridges, regular inspection of special equipment such as boilers and elevators. In the rescue and disposal process, nondestructive testing technology can be used to assist in the investigation and personnel search and rescue of accident or disaster site, so as to improve the efficiency of rescue and disposal and reduce casualties.
Prevention of accident and disaster
01 storage and transportation facilities of hazardous chemicals
The enterprises and storage and transportation facilities of hazardous chemicals are widely distributed in cities. Once an accident occurs, it will cause serious harm to people's lives and properties, which is the focus of emergency management departments at all levels.
For the corrosion, crack and weld defects of oil and gas transportation pipeline, ultrasonic guided wave detection can be adopted. Its advantages are long detection distance and high detection accuracy. Different modes and frequencies can be used to detect different types of defects. In the field of pipeline detection, the magnetic leakage detection or conventional ultrasonic detection is used to judge the thinning or deformation of the pipe wall. Its advantage is that it is not affected by the external environment of the pipeline, and can be used for the detection of submarine pipelines or buried pipelines. Fiber optic detection technology is very suitable for disturbance monitoring along the pipeline, and can be used for monitoring theft and construction interference.
In view of the corrosion and crack of the tanks of dangerous chemicals such as oil and gas, conventional ultrasonic detection and magnetic leakage detection technology are usually used, and the bottom plate and wall plate of the tank are comprehensively scanned by means of moving devices such as climbing robot. Some scholars have studied the large area nondestructive testing of tank floor by using ultrasonic Lamb wave, and got a good detection effect. Acoustic emission technology is usually used for on-line monitoring of oil and gas tanks in service, but it is easy to be interfered by external factors, and the accuracy of detection needs to be improved.
02 city lifeline
The lifeline of a city is not only related to the daily life of the people and the normal operation of the city, but also threatens the health of the surrounding people after the accident.
For the detection of water supply, drainage, heating and gas supply pipelines, the research on related aspects at home and abroad is also early, and the defect detection methods are relatively mature. The commonly used detection methods include eddy current, ray, ultrasonic, magnetic leakage and ultrasonic guided wave. For the corrosion defects of large box culvert sewage pipes poured with concrete, ultrasonic phased array and ground penetrating radar can be used to detect the corrosion defects.
In rail transit, ultrasonic, eddy current, magnetic leakage and other methods are usually used to non-contact nondestructive testing of in-service rail. Different detection methods are suitable for different positions and types of defects. These systems can be carried out on special vehicles for mobile detection, with a speed of up to 80km / h. In view of the corrosion monitoring of the stray current of urban subway on the metal structures such as gas pipeline and water supply pipeline along the line, as well as the monitoring of the settlement of the subway dome and the influence of the construction along the line, optical fiber sensing technology can be used.
03 special equipment
Special equipment testing is one of the key application directions of nondestructive testing technology. Among special equipment, "boiler, pressure vessel (including gas cylinder), pressure pipe" are called pressure bearing special equipment. It is mostly in high temperature and high pressure operating environment, which has a large operating load, which is prone to aging, corrosion, cracking and other defects, which has a great potential danger. According to the regulations on safety supervision of special equipment, the nondestructive testing methods for pressure bearing special equipment are mainly magnetic particle testing, ray testing, eddy current testing and ultrasonic testing, and portable equipment is often used for testing. Some scholars have also studied the defect detection of special pressure equipment by means of ultrasonic guided wave technology, electromagnetic detection technology and acoustic emission technology.
"Elevator, lifting machinery, passenger ropeway, large amusement facilities and special motor vehicles in the yard (factory)" are called mechanical and electrical special equipment. Elevator, passenger ropeway and large amusement facilities are common in cities and scenic spots. Their operation safety is closely related to the life and entertainment of ordinary people. Steel wire rope is an important bearing member of mechanical and electrical special equipment, and its safety performance has been widely concerned. For the nondestructive testing of wire ropes, the most common method is electromagnetic detection technology, which can not only detect the broken wire and other defects, but also can detect the stress concentration and fatigue damage. The conventional testing methods such as ultrasonic and ray are often used for the detection of other parts in mechanical and electrical special equipment.
04 construction engineering structure
The construction engineering structure is the basic element of the city, and the main carrier of the mass production and life. The safety management of the construction engineering structure is also an important content of the urban public security.
Acoustic emission technology plays an important role in health monitoring of bridges, dams, tunnels and other structures. In the process of using these structures, microcracks will be generated under external loads. The generation and growth of microcracks will release energy, and spread in the form of elastic waves. The receiving sensors can capture these signals. The safety status of the monitored structure can be obtained through analysis. Ultrasonic testing, surface wave detection, infrared detection and other nondestructive testing methods can also be used in the construction engineering structure detection in specific situations.
In modern cities, glass curtain wall widely exists in high-rise buildings is also one of the focus of nondestructive testing. For the failure detection of structural adhesive of glass curtain wall, ultrasonic detection is the most widely used, and acoustic emission, ray and infrared detection methods are also reported.
05 others
The process of limited space operation in cities also requires nondestructive testing technology to provide safety assurance. Due to the poor natural ventilation, the limited space is easy to cause toxic and harmful, flammable and explosive gas volume accumulation and air oxygen content reduction. In recent years, the limited space operation accidents occur frequently, one of the reasons is the lack of safety detection means and incomplete gas detection equipment. If the gas condition in the limited space is detected by gas detector before operation and the harmful gas and oxygen concentration monitoring is carried out continuously during the operation, it will greatly reduce or even prevent similar accidents.
Fire is one of the most important accidents and disasters in the field of urban public safety. NDT technology has also been applied in fire monitoring. Smoke detector and temperature sensor are common sensor types in fire monitoring, and also belong to the category of non-destructive testing.